Existing satellite broadcast communication systems, such as, for example, that currently utilized by Sirius Satellite Radio, employ two forms of modulation to convey information, namely, single carrier Quadrature Phase Shift Keying (QPSK) and multicarrier differential Coded Orthogonal Frequency Division Multiplexing (COFDM).
QPSK is a modulation technique that allows for the transmission of digital information across an analog channel. Data bits are grouped into pairs with each pair represented by a particular waveform, commonly referred to as a symbol. There are four possible combinations of data bits in a pair, and a unique symbol is required for each possible combination of data bits in a pair. QPSK creates four different symbols, one for each pair, by changing the I gain and Q gain for the respective cosine and sine modulators. The symbol is then sent across the analog channel after modulating a single carrier. A receiver will then demodulate the received signal and look at the recovered symbol to determine which combination of data bits was sent.
COFDM is a modulation technique that distributes a single digital signal across 1,000 or more signal carriers simultaneously. Coded data is modulated and inserted into orthogonal carriers in the frequency domain. Because signals are sent at right angles to each other, the signals do not interfere with one another. Multi-path effects describes the scattering of a signal due to obstructions such as, for example, canyons, buildings, etc., which can introduce distortions and “ghosting.” Multi-path effects can cause a signal to take two or more paths to reach its final destination. COFDM is highly resistant to multi-path effects because it uses multiple carriers to transmit the same signal.
Existing satellite broadcast communications systems place a key synchronization signal (known as a cluster synchronization signal) in a fixed frequency domain COFDM bin location, such as, for example, the first of the 1,000 or more signal carriers. That is, in such systems, the cluster synchronization signal does not use any of the significant frequency diversity that is available in a COFDM system. An unfortunate consequence of this scheme is that when a receiver, such as a satellite radio, for example, experiences a constant or slow moving multi-path null in the vicinity of this frequency domain bin, detection of the key synchronization signal is degraded and often impossible to detect. This problem is known as “slow speed mute.” Because proper detection of the cluster synchronization signal is critical for proper signal acquisition and decoding, slow speed mute can cause a complete loss of all possible data reception.
In some systems, slow speed mute is currently addressed by employing transmitter diversity at each COFDM repeater site utilizing a second broadcasted signal as a copy of the first COFDM signal, but offset in frequency by 40 Hz and time delayed by 1 μs. The resulting signal at every receiver will at least be a sum of the original signal and such second offset signal of essentially the same power (scaled according to the path gain of each transmit signal, which is usually equal, as well as the normal channel reception characteristics of each receiver). This transmitter diversity scheme causes a self induced time varying multi-path across the entire spectrum of the COFDM signal (i.e., as if a 1 μs, 0 dB down reflection of the original signal existed). Because the transmitter cannot determine the phase angle of arrival of each receiver—and, thus, will not know if the self induced multi-path is possibly adding to, or ameliorating, a slow speed mute problem—the frequency offset on the second signal ensures that a phase rotation of the second received signal will “roll” the self induced multi-path nulls across the received spectrum at a rate of the frequency offset of 40 Hz. This time varying signal ensures that the cluster sync bin “sees” a time varying signal in its respective (Fast Fourier Transform) FFT bin, essentially averaging out any fixed multi-path null caused by the channel. This induced multi-path makes the FFT cluster bin much less likely to be “stuck” in a multi-path null, providing a method for detecting the key cluster bit synchronization signal. Unfortunately, the purposely induced multi-path not only affects the cluster sync bin but each FFT bin that comprises the COFDM signal as well. The self induced multi-path is 0 dB down in amplitude, which reduces the entire link margin of the signal. This effect creates a lack of coverage area due to the now combined effect of signal plus self induced multi-path distortion.
What is needed in the art is an alternative implementation of diversity transmission that can address slow speed mute and at the same time overcome or ameliorate the problems of such conventional systems and techniques.